There is known a storage system that has a plurality of disk devices and a storage controller for controlling these disk devices, wherein each of the disk devices functions to create a parity (Patent Literature 1, for example). In this storage system, the storage controller transmits updated data (new data) to a disk device storing data before update (old data). From the old data and the new data, the disk device creates an intermediate value (intermediate parity) used for creating a new parity, writes the new data into the disk thereof, and empties the storage region in which the old data was stored, to have a blank region. The disk device transmits this intermediate parity to the storage controller. The storage controller stores the intermediate parity in a cache memory and nonvolatile memory of the storage controller. The storage controller then reads the intermediate parity from the cache memory and transmits the intermediate parity to a disk device storing an old parity. This disk device creates a new parity from the received intermediate parity and the old parity, writes the new parity into the disk thereof, and empties the storage region in which the old parity was stored, to have a blank region.